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Award Ceremony Speech

Von Laue's epoch-making
discovery of the diffraction of the X-rays in crystals, on the
one hand established wave motion as the essential quality of
those rays and, on the other, afforded the experimental proof of
the existence of molecular gratings in the crystals. The problem,
however, of calculating the crystal structures from von Laue's
formulae was an exceedingly complicated one, in as much as not
only the space lattices, but also the wavelengths and the
intensity-distribution over the various wavelengths in the
spectra of the X-rays, were unknown quantities. It was
consequently a discovery of epoch-making significance when W.L.
Bragg found out that the phenomenon could be treated
mathematically as a reflection by the successive parallel planes
that may be placed so as to pass through the lattice points, and
that in this way the ratio between the wavelengths and the
distances of the said planes from each other can be calculated by
a simple formula from the angle of reflection.

It was only by means of that simplification of the mathematical
method that it became possible to attack the problem of the
crystal structures, but to attain the end in view it was further
necessary that the photographic method employed by von Laue
should be replaced by an experimental one, based on the
reflection principle, which admitted of a definite, even though
at first unknown, wavelength being made use of. The instrument
requisite for the said purpose, the so-called X-ray spectrometer,
was constructed by Professor W.H. Bragg, W.L. Bragg's father, and
it has been with the aid of that instrument that father and son
have carried out, in part conjointly, in part each on his own
account, a series of extremely important investigations
respecting the structure of crystals.

If a number of cubes are laid on and beside each other in such a
way that one cube face coincides in every case with the face of
an adjoining cube, whereby consequently eight vertices always
meet in one point, those angular points give a visual picture of
the lattice points in the so-called simple cubic lattice. If
again a lattice point is placed so as to coincide with the
central point of each cube face, the so-called face-centred cubic
lattice is obtained, whereas the centred cubic lattice has one
lattice point in every cube-centre. With the exception of these
three cases there is no cubic lattice that fulfils the condition
that parallel planes placed in any direction whatever so as to
pass through all the lattice points, shall also be at a constant
distance from each other. The space lattice in the regular or
cubic system must therefore coincide with one of those three, or
constitute combinations of them. In such lattice combinations, on
the other hand, in which the condition just mentioned is not
fulfilled, where consequently parallel planes placed to pass
through all the lattice points in certain directions are not
equidistant, that circumstance is revealed by an abnormal
intensity distribution among spectra of different orders, when
the reflection takes place by those planes.

From crystallographical data it is always known how the face of a
cube is situated in any given regular crystal, and there is
consequently no difficulty in fixing the crystal on the
spectrometer table in such a way that the reflection shall take
place by planes with any prescribed orientation.

The rays falling on the crystal were produced by X-ray tubes,
platinum being at first used for the anticathode. The
characteristic X-radiation of the metals consists, as is well
known, of a few strong lines or narrow bands, and the very first
experiments with the spectrometer revealed the X-radiation that
is characteristic of platinum. However, in the research
undertaken to find out the nature of complicated space lattices,
in which an abnormal intensity distribution among spectra of
varying orders constitutes one of the most important of the
results observed, it soon proved desirable to have available an
X-radiation of approximately half the wavelength of the strongest
platinum-line. From theoretical considerations W.H. Bragg
regarded it as probable that a metal whose atomic weight was
somewhere near the figure 100, would give a characteristic
radiation of the desired wavelength. Accordingly anticathodes of
palladium and rhodium were produced, which fully answered the
purpose in view, so that spectra ev en of the fifth order could
be obtained and measured. In order to take practical advantage,
however, of those results, it was essential to have a method for
calculating the intensity in the case of a complicated space
lattice, that would prove simpler than the one given by von
Laue's theory, and W.L. Bragg developed one.

The above is a brief sketch of the methods discovered by the two
Braggs for investigating crystal structures. The results of their
investigations embrace a large number of crystals belonging to
various systems and can only be cursorily summarized in this
place.

To begin with, the two investigators applied themselves to the
simplest types of the regular system, represented by the alkaline
haloid salts. It then proved that potassium bromide and potassium
iodide showed the spectra that are characteristic of a
face-centred cubic lattice, while the spectra of potassium
chloride represented a simple cubic lattice, sodium chloride
occupying an intermediate position. As it must be assumed, on the
strength of the analogy of these salts, both in a chemical and a
crystallographical sense, that they are possessed of a
corresponding space lattice, which could also be corroborated in
another way, it was proved by those researchers that the lattice
of the crystals in question consists of two face-centred cubic
lattices corresponding to the two atoms, which interpenetrate in
such a way that they together constitute one single cubic
lattice.

From these investigations it follows that a metal atom in the
crystals of the alkaloid salts is situated at one and the same
distance from the six haloid atoms nearest to it, and vice versa
- a relationship that was found to prevail, mutatis mutandis, in
all the crystals examined. That means the exceedingly important
discovery, both for molecular physics and chemistry, that the
crystals consist of atomic lattices and not, as has been always
imagined, of molecular ones.

Two face-centred cubic lattices can also interpenetrate in such a
way that every point belonging to the one lattice is at the
centre of gravity of a tetrahedron whose vertices are points
belonging to the other lattice. That structure was found by the
two Braggs in the diamond, and afforded an experimental support
for the tetrahedral arrangement that chemists postulate for the
four-coordinate carbon. On the other hand, the explanation became
evident of why crystallographers have not been able to agree
regarding the class in the regular system to which the diamond
should be referred.

It would carry us too far and be quite too complicated a
proceeding to give an account here of the further investigations
into the space lattices of the crystals. It will suffice to add
that, in the course of their investigations, the two Braggs have
also discovered important relations between the amplitude and the
phase difference of the diffracted rays on the one hand and the
atomic weights on the other, and have also shown experimentally
the influence of heat on the space lattice.

Finally it may be mentioned that the two investigators have also
determined the wavelengths of the X-rays and the distances
between the successive planes placed to pass through the lattice
points with such exactitude, that the error, if any, is probably
a t most some few units per cent and is more due to the general
physical constant entering into the calculations than to the
measurements themselves.

Thanks to the methods that the Braggs, father and son, have
devised for investigating crystal structures, an entirely new
world has been opened and has already in part been explored with
marvellous exactitude. The significance of these methods, and of
the results attained by their means, cannot as yet be gauged in
its entirety, however imposing its dimensions already appear to
be. In consideration of the great importance that these methods
possess for research in the realm of physics, the Swedish Royal
Academy of Sciences decided that the 1915 Nobel Prize in Physics
should be divided between Professor W.H. Bragg and his son W.L.
Bragg, in recognition of their services in promoting the
investigation of crystal structures by means of X-rays.